Portable combustion powered tools for use in driving fasteners into workpieces are described in commonly assigned patents to Nikolich, U.S. Pat. Re. No. 32,452, and U.S. Pat. Nos. 4,552,162, 4,483,473, 4,483,474, 4,403,722, and 5,263,439, all of which are incorporated herein by reference. Similar combustion powered nail and staple driving tools are available commercially from ITW-Paslode under the IMPULSES.RTM. brand.
Such tools incorporate a generally gun-shaped tool housing enclosing a small internal combustion engine. The engine is powered by a canister of pressurized fuel gas, also called a fuel cell. A battery-powered electronic power distribution unit produces the spark for ignition, and a fan located in the combustion chamber provides for both an efficient combustion within the chamber, and facilitates scavenging, including the exhaust of combustion by-products. The engine includes a reciprocating piston having an elongate, rigid driver blade disposed within a piston chamber of a cylinder body.
The wall of a combustion chamber is axially reciprocable about a valve sleeve and, through a linkage, moves to close the combustion chamber when a workpiece contact element at the end of a nosepiece connected to the linkage is pressed against a workpiece. This pressing action also triggers a fuel metering valve so as to introduce a specified volume of fuel gas into the closed combustion chamber from the fuel cell. The metering valve may take the form of a solenoid valve, which is powered by the battery, or may be a purely mechanical valve.
Upon the pulling of a trigger, which causes the ignition of a charge of gas in the combustion chamber of the engine, the piston and driver blade are shot downward so as to impact a positioned fastener and drive it into the workpiece. As the piston is driven downward, a displacement volume enclosed in the piston chamber below the piston is forced to exit through one or more exit ports provided at the lower end of the cylinder. After impact, the piston then returns to its original, or "ready" position through differential gas pressures within the cylinder. Fasteners are fed into the nosepiece from a supply assembly, such as a magazine, where they are held in a properly positioned orientation for receiving the impact of the driver blade. The power of the tools differs according to the length of the piston stroke, the volume of the combustion chamber, the fuel dosage and similar factors.
The combustion powered tools have been successfully applied to large workpieces requiring large fasteners, for framing, roofing and other heavy duty applications. Smaller workpiece and smaller fastener trim applications demand a different set of operational characteristics than the heavy-duty, "rough-in", and other similar applications.
One operational characteristic required in trim applications is the ability to predictably control fastener driving depth. For the sake of appearance, some trim applications require fasteners to be countersunk below the surface of the workpiece, others require the fasteners to be sunk flush with the surface of the workpiece, and some may require the fastener to stand off above the surface of the workpiece. Depth adjustment has been achieved in pneumatically powered and combustion powered tools through a tool controlling mechanism, referred to as a drive probe, that is movable in relation to the nosepiece of the tool. Its range of movement defines a range for fastener depth-of-drive. Exemplary depth adjustment tool-controlling mechanisms are disclosed in Volkmann U.S. Pat. No. 3,519,186, Canlas, Jr., U.S. Pat. No. 4,767,043, Mukoyama U.S. Pat. No. 5,219,110, and Johnson, Jr., U.S. Pat. No. 5,385,286.
Another depth-of-drive adjustment having a spool on a thread for adjusting depth is disclosed in commonly assigned U.S. Pat. 5,685,473. The spool has ribs that engage a spool restraining element when the tool controlling mechanism is pressed inwardly toward the tool body. This prevents spool movement when the tool is enabled for firing. In these prior depth adjustment mechanisms, the operator typically obtains a desired depth through trial and error. If the depth is altered for some reason, it may take additional trial and error to return to a previously used depth. This experimentation to obtain a desired depth slows the operation of the tool, and may result in workpiece damage in trim applications which require a precisely controlled depth.
Another difficulty in trim applications relates to the small fasteners used. Typically, these fasteners are fed into the nosepiece by a magazine which is angularly mounted below the handle used by an operator to grip and trigger the gun. They are advanced by a spring loaded fastener pusher, generally similar in operation to those found in staplers. It is preferable to prevent firing when a certain number of fasteners are remaining, or when the fasteners are exhausted, but the resulting movements created in the magazine by trim sized fasteners provide very little mechanical feedback due to their small size. For example, typical finishing brads loaded into a magazine move in increments of about 0.060" (1.5mm). As a result, a lockout bar moving with the brads is unable to block the larger diameter drive probe and related linkage, used to enable firing, upon the movement induced after the driving of a small diameter brad.
The fasteners used in trim applications may also be difficult to manipulate and load due to their small size. Rear loading, top loading, and side loading arrangements are known in the art. The side and top loading arrangements are more mechanically complex since the direction in which the fasteners are loaded into the magazine is not in the same direction into the nosepiece that the fasteners travel during operation. The known rear loading arrangement is more easily implemented, but is more difficult to use because no portion of the gun provides a guiding surface for a user to align a fastener, or the beginning of a group of fasteners, with the opening for placing fasteners into the magazine.
User ergonomics and tool balance also play a more pronounced role in trim applications. Manipulation of the tool to fasten horizontally disposed trim pieces and trim pieces in awkward positions results in user fatigue, which is amplified by a tool which is not balanced around the user grip area. Typical combustion tools have the handle disposed away from the axis of the driver blade so as to accommodate the fuel cell held in the housing at a point adjacent to the termination of the handle at the housing. This results in a natural tendency of the tool to lean away from the user when gripped at the handle since most of the tool's weight is centered near the axis of the driver blade. Recoil is also pronounced since the distance between the handle and the driver acts as a moment arm. A user must oppose these forces when using the tool, resulting in fatigue.
User comfort is also affected by tool weight and stability. Typically, the fastener driving tools are held together by numerous screws and rivets at various points around the tool's periphery. This increases weight and decreases rigidity. Since the magazine and tool housing are separate pieces, the separate fastening also can lead to alignment problems in delivering fasteners into the nosepiece. These operational problems are separate from additional assembly problems related to the same typical tool features, which make assembly more difficult and expensive.
Trim applications also require more exacting positioning during firing. Typical tools obscure an operator's sight line since the body of the tool interferes with the view to a portion of the workpiece proximate to the point at which the fastener will be driven into the workpiece.
Tools having self contained power sources generally must also be portable, and cost is an important concern. To keep cost and weight down, many portable tools lack an on/off switch. To prevent unintentional operation of the tool, some operators disengage the battery held in the handle or some other portion of the tool. In combustion tools, the battery is necessary to produce the spark and fan movement necessary to tool operation. If the operator is moving, for instance by climbing a ladder or scaffold, the loosely held disconnected battery (or batteries) may fall out. This is an inconvenience to the operator, a cause of damage to the battery, and a potential hazard to the operator and anyone below the operator.